This invention relates to signal processing and, more particularly, to digital circuitry for converting digital data to signal sequences suitable for linear time-division-multiple-access (TDMA) systems.
In a typical TDMA radio system, multiple users are respectively assigned time slots in a specified channel. During each time slot, the assigned user generates a burst of signals for transmission. To minimize the extent of so-called guard time between adjacent slots, it is desirable to control each burst to turn ON and OFF in a relatively rapid manner. At the same time, the ON/OFF control of each burst in a channel must be done in a manner that does not result in spectral spreading that can interfere with other channels that are nearby in frequency.
In theory, signaling pulses such as square-root Nyquist pulses or Nyquist pulses are well suited for representing signals in a TDMA burst. By means of such a representation of overlapping pulses, a burst characterized by zero interpulse interference within the burst can be realized. By preserving the Nyquist pulse shape at the beginning and end of each burst, transition times between adjacent bursts can be minimized while causing no additional spectral spreading.
Various approaches have heretofore been suggested for generating bursts that consist of overlapping Nyquist pulses. Some of these approaches require the use of relatively complicated and expensive filters. Other approaches have attempted to utilize digital circuitry to generate the requisite pulses. But these latter approaches have not resulted in relatively simple implementations that represent the beginning and end portions of each burst in a manner that addresses the spectral spreading problem.
Accordingly, efforts have continued by workers skilled in the art directed at trying to devise relatively simple and inexpensive ways of accurately generating bursts that consist of overlapping Nyquist pulses without distortion in the ON/OFF periods. It was recognized that these efforts if successful would contribute importantly to the design of improved TDMA systems.
Various aspects of TDMA have been treated in prior art patents such as U.S. Pat. No. 4,849,991, entitled "Method and Circuitry for Determining Symbol Timing for Time Division Multiple Access Radio Systems," issued Jul. 18, 1989 to the inventors herein; U.S. Pat. No. 4,937,841, entitled "Method and Circuitry for Carrier Recovery for Time Division Multiple Access Radio Systems," issued Jun. 16, 1990 to J. C. Chuang and N. R. Sollenberger, the latter an inventor herein; and U.S. Pat. No. 4,941,155, entitled "Method and Circuitry for Symbol Timing and Frequency Offset Estimation in Time Division Multiple Access Radio Systems," issued Jul. 10, 1990, also to J. C. Chuang and N. R. Sollenberger. A technique for jointly performing burst synchronization and error detection in a TDM/TDMA system is described in co-pending commonly assigned patent application Ser. No. 404,946, filed Sep. 9, 1989. Also, a technique for assigning up-link frequencies to the portables in a TDM/TDMA system is described in co-pending commonly assigned patent application Ser. No. 619,059, filed Nov. 28, 1990.